GB2240944A

GB2240944A - Enlarging metallic tubes

Info

Publication number: GB2240944A
Application number: GB9101966A
Authority: GB
Inventors: Noel James Parratt; Kevin David Potter
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1990-02-20
Filing date: 1991-01-30
Publication date: 1991-08-21
Anticipated expiration: 2011-01-30
Also published as: GB2240944B; GB9101966D0; GB9003826D0

Abstract

An aluminium or high strength aluminium alloy tube 12 has a portion 10 radially expanded outwardly into a cavity 18 of a conforming mould 16 by application of pressure on the inside of the workpiece wall by a plunger having a rubber core 26 and an outer sleeve 28. The plunger is compressed axially by a ram 30. The sleeve 28 is of tough rubber reinforced with an orientated fabric reinforcement 32 of substantially inextensible fibres which may for example be woven, braided or helically wound. The reinforcement 32 controls deformation of the sleeve 28 during radial expansion thereof such that the sleeve shortens axially together with the tube 12. The outer surface of the tube 12 is lubricated to reduce friction between the tube and the mould 16. The tube may be preheated prior to being placed in the mould. in an alternative embodiment, gas or hydraulic pressure is applied to the interior of a sleeve (28, Fig 6). <IMAGE>

Description

ENLARGING METALLIC TUBES This invention relates to a process and apparatus for enlarging, ie radially expanding to a predetermined profile, a portion of a tubular metallic workpiece having a stress/strain characteristic in which it is plastic at tensile failure. Examples of materials having such a characteristic are aluminium and certain aluminium alloys.

Among the various known methods for obtaining local radial expansion of generally tubular components are those of cold working by spinning or swaging. Spinning is quite satisfactory for many applications where sharp radii are not required, and where the substantial thinning of the tube wall which is inherent in the spinning process does not matter. Conventional swaging processes are generally only satisfactory for tubes with quite thin walls, and even then there is inevitably substantial further thinning of the wall during swaging.

where a tubular component is required to have an axial cross section of substantially constant wall thickness and/or bends of small radius between successive sections of the component, it can be either fabricated in sections or made by casting. However, both of these methods - particularly fabrication - are comparatively expensive and time-consuming, while for many applications the strength of cast components is insufficient unless they are to be unacceptably heavy. The casting approach is thus not generally open to the manufacturer who wishes to make a very strong but very light component, for example for aerospace use.

In an aerospace application, the component may for example be part of a rocket motor, calling for high mechanical strength over a wide range of temperature and pressure conditions, and the ability to withstand a high applied hoop stress. For such an application, it is common to use high tensile aluminium alloys, and the invention is especially applicable to such alloys.

In order to achieve a tubular component that has a diameter varying along its length for operating under such conditions, it is essential that the forming process does not reduce the wall thickness significantly. Where adjacent sections of the tube are joined by bends of sharp radius, not only must the wall thickness be maintained, but there must be no tendency to stress cracking at these bends, either during the forming process or subsequently when the component is in use.

One method that has been tried is explosive forming, which has the serious disadvantage of being rather dangerous and therefore calling for costly and tiresome precautions.

Another method that has been proposed for deforming ductile metal tubes is to place the tube in a mould and pressurise the inside of the tube, either with a suitable fluid applied directly, or with a deformable bladder or cylinder which is internally pressurised in some way and which expands radially to force the tube walls to conform with the profile of a mould in which it is mounted.

It has also been proposed to apply the necessary hydraulic pressure to force the tube walls into the mould profile by a "dry" mechanical process in which endwise pressure is applied by a ram to a rubber cylinder or plug, fitted into the tube which is itself fitted in the mould. The applied pressure deforms the rubber plug and, again, deforms the tube wall into the mould profile.

where such methods are used, the amount of tube deformation that is possible before the tube fails is very limited. Typically no more than 20% strain can be achieved, because it is at about this level of strain that the stress/strain characteristic becomes unstable. In addition, in the zone being deformed, substantial thinning of the wall is found to take place, especially at the bends, due to the increase in the surface area of the tube which is inherent in any process for enlarging a tube. The bends thus become points of particular weakness.

According to a first aspect of this invention, a process for radially expanding to a predetermined profile a portion of a tubular metallic workpiece, having a stress/strain characteristic in which it is plastic at tensile failure, includes: - inserting a plunger comprising a cylindrical sleeve element into the workpiece, to extend through said workpiece portion with the sleeve element adjacent to the workpiece bore, the sleeve element being of rubbery material and including an orientated fibrous reinforcement such as to control the deformation of the sleeve element during the subsequent radial expansion thereof, such that the latter is axially shortened in a predetermined relationship with increase in its diameter; - inserting the workpiece into a hollow mould having an internal profile corresponding to the said predetermined profile; and - applying hydraulic pressure to the workpiece bore through the plunger sleeve element, under conditions such that friction between the workpiece and the mould is substantially smaller than that between the workpiece and the sleeve element, thereby causing the sleeve element and workpiece together to expand radially and shorten axially until the profile of the workpiece conforms with that of the mould.

The process may be performed hot or cold, depending on the metal or alloy used, and provided the ductility of the workpiece is high enough to permit adequate deformation. where it is of a high tensile aluminium alloy having low ductility at ambient temperatures, the workpiece is preferably heated to about 3500C, and is worked while still hot enough to maintain sufficient ductility.

It is of course inherent in this that the alloy is in the unstable part of its stress/strain curve while being expanded radially.

However, as noted above, the material of which the sleeve element is made is one in which stress increases continuously (though not necessarily in a straight line) with strain.

The suitable orientated fibrous reinforcement gives the sleeve a very high stiffness in certain directions and very low stiffness in other directions. Due at least to these features the sleeve element provides sufficient mechanical support to the portion of the tube which is being locally deformed at all times. This gives a significant measure of protection against tensile failure of the tube material. It is found that using the method of the invention, the tube wall radius can be locally expanded successfully, and without tensile failure or stress cracking, by considerably more than with the prior art methods. For example, with aluminium exhibiting, when unsupported, a maximum hoop strain of 32% at failure, the equivalent figure using the process of this invention was 50%.With heated high tensile aluminium alloys worked at about 350 C, this process has successfully achieved 60% hoop strain.

Thinning of the tube wall is found to be no more than 20%.

Using suitable lubrication to reduce the friction between the tube wall and the mould virtually to zero and ensuring that the coefficient of friction between the sleeve element and the tube wall is substantially unity (so that there is no slip between them), it is possible to reduce the amount of wall thinning almost to zero.

This is true even at sharp bends in the tube wall, and is a function of the geometry chosen for the reinforcing fibres.

The "no slip" criterion can be largely achieved by ensuring that the outer surface of the sleeve element, for a given material of the latter, has a suitable coefficient of friction with the workpiece material. The sleeve element in effect grips the tube, and with absence of friction between the tube and the mould, the sleeve element and tube are able to deform as one piece, so that as the sleeve element is deformed outwardly by the hydraulic pressure (and thus becomes reduced in axial length) the metal of the tube wall shortens with it, deforming in plastic flow as it does so.

It is also found that the process leads to very fast operation, eg less than 10 seconds.

According to the invention in a second aspect, apparatus for radially expanding to a predetermined profile a portion of a tubular metallic workpiece comprises: - a hollow mould having an internal profile corresponding to the said predetermined profile; - a plunger comprising a cylindrical sleeve element of rubbery material, including an orientated fibrous reinforcement such as to control the deformation of the sleeve element during radial expansion thereof under internal hydraulic pressure such as to cause axial shortening of the sleeve element, by controlling the relationship between shortening and radial expansion of the sleeve element in a predetermined manner, the sleeve element being adapted for insertion in a said workpiece; and - pressurising means for applying such internal hydraulic pressure to the sleeve element, whereby to deform such a workpiece in frictional contact with the sleeve element by simultaneous radial expansion and axial shortening of the said workpiece portion and sleeve element together.

The reinforcement is an essential feature of the invention. It enables the sleeve portion to perform its function of giving adequate support to the tube wall with increasing tensile stress.

An unreinforced component made from any rubber of the toughness needed to deform metal tube, will not be stiff enough to obtain the required degree of control of the deformation of the metal. The reinforcement also controls the change of surface area of the rubber during deformation.

The fibres of which the reinforcement is made are preferably orientated in two directions, such that the reinforcement can deform into a configuration having an increased surface area as the rubber in which it is embedded expands. Preferably therefore, the fibres of the reinforcement are orientated in two directions. The fibres themselves, in order to provide satisfactory reinforcement, are individually substantially inextensible. They may be of any suitable material, for example tyre cord, steel wire, or a suitable synthetic polymeric fibre. The reinforcement may conveniently be a cloth fabric.This may be in a form, typically woven or braided, in which the total length of each fibre is inextensible; alternatively it may for example be wound helically on the sleeve element. the relative orientation between the fibres orientated in one direction and those orientated in another (ie between the warp and weft in the case of a woven fabric) will be such that the surface area defined by the fabric is much smaller than the maximum possible. This enables the fibres to reorientate themselves to give a larger surface area as the sleeve element expands.

where the reinforcement is of substantially inextensible construction, the sleeve element is a separate, hollow, cylindrical sleeve member. The pressurising means then preferably comprises a separate, lubricated core or plug of rubbery material which is fitted within the sleeve element (the core and sleeve element then constituting the plunger), together with some means such as a ram for applying endwise pressure to the core, so as to squeeze it against the inside of the separate sleeve element. It is however also possible to fill the hollow interior of the sleeve element with a hydraulic liquid or gas to apply the hydraulic pressure, provided the ends of the assembly are suitably sealed.

Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings (all of which are very diagrammatic), in which: Figure 1 is a transverse cross section of a first embodiment of swaging apparatus according to the invention immediately before the application of pressure to expand a portion of a tubular workpiece; Figure 2 is a section on the line II-II in Figure 1; Figure 3 is a view similar to Figure 1 but on a larger scale, showing one half of the apparatus at the end of the swaging operation Figure 4 shows a portion of the sleeve element, viewed radially but developed flat, with a woven, braided or helically-wound reinforcement shown in its relaxed state; Figure 5 shows the same portion of the sleeve element when fully expanded radially; and Figure 6 shows the apparatus adapted for pressurisation by a fluid.

Referring first to Figure 1 to 3, the apparatus is designed to expand a portion 10 of a workpiece 12 to the profile shown at 14 in Figure 3. The workpiece in this example is a simple tube of a high strength aluminium alloy. The apparatus comprises a hollow mould 16, split into two halves as seen in Figure 2 in the conventional way, and having an internal profile which includes an annular cavity 18 corresponding to the profile 14 which is required for the tube portion 10. The top end of the mould is closed by an end plate 20 carrying a support bar 22 extending coaxially through the middle of the mould. The top end of the workpiece abuts against the end plate 20 as seen in Figure 1, as does the top end of a plunger generally indicated at 24.

The plunger consists of a central core of soft silicone rubber, and a separate sleeve member 28 which is fitted around the core 26. A vertically acting ram 30 engages the bottom end of the plunger.

The sleeve 28 is made of a suitable very tough silicone rubber, and is reinforced with an orientated fibrous reinforcement 32 (Figure 3), which is very securely attached to the rubber, preferably by being moulded into it. This reinforcement extends over substantially the whole length and circumference of the sleeve 28 and is generally coaxial with the rubber, so that the whole of the sleeve is reinforced in a uniform manner, symmetrically about its axis.

The purpose of the reinforcement 32 is to control the deformation of the sleeve 28 when, as will be seen below, the latter is expanded radially. To this end, the reinforcement consists of yarns or threads or any material suitable for this purpose; a preferred material is KEVLAR (Trade Mark) fibre. The material is preferably, in any case, one that is substantially inextensible, though some degree of predictable extension may be permissable.

The reinforcement 32 may be wound helically on or in the rubber of the sleeve 28. Although the resulting orientation may be in only one direction, the reinforcing fibres are preferably orientated in two directions. These two directions are preferably opposed helical directions, ie clockwise and anti-clockwise about the axis of the sleeve. With the wound construction mentioned above, the reinforcement would then simply consist of yarn, or several parallel yarns, wound helically in one direction and overlaid with a further yarn, or several parallel yarns, wound helically in the opposite direction.

An alternative form of the reinforcement 32 is that of a woven or braided cloth. Whether it is in this form or in the simple helically-wound form, the reinforcement may be of two-ply construction, in which it consists of two layers of cloth or two sets of helical windings.

Although the reinforcement is shown in Figure 3 at the outer surface of the sleeve 28, it may be incorporated within the thickness of the rubber.

The reinforcement 32 seen in the small developed section 29 of the sleeve 28 that is represented in both Figures 4 and 5 may be taken to represent bidirectionally, helically-wound reinforcement or woven or braided cloth reinforcement. These two Figures are drawn to the same scale, with one axial plane of the sleeve indicated at 36. In the relaxed or "as made" state (Figure 1), in which the sleeve section 29 is shown in Figure 4, the fibres of the reinforcement 32 are orientated at a comparatively small acute angle a with respect to the plane 36. Thus, with bidirectional winding as shown, the two sets of yarns (warp and weft in the case of a woven cloth) are oblique to each other.

In operation, the tube 12, Figures 1 and 2, is inserted in the mould 16 so as to bridge the cavity 18, and with a suitable lubricant between them so as to reduce the coefficient of friction between the outer surface of the tube and the inner surface of the mould substantially to zero. The plunger 24 is inserted in the tube, to extend through the portion 10 and so that the sleeve 28 lies adjacent to the bore of the tube. The support bar 22 then extends through a central hole in the plunger core 26, providing mechanical support for the latter. It should be noted that in some applications this mechanical support may be found unnecessary, in which case the bar 22 may be omitted and the core can then be made solid.For some metals and alloys, for example extruded aluminium, it may be possible to perform the process at room temperature, where there is sufficient ductility at the working temperature. For other materials, and particularly certain high strength aluminium alloys, the process must be performed at a higher temperature, for example in the range 300 to 3500C. where the workpiece is to be hot worked, therefore, it is preheated to the required temperature, by any suitable means, immediately before being inserted in the mould 16, the plunger 24 and ram 30 then being positioned, and operated as will be described below, while the tube 12 is still within the required temperature range.

The ram 30 is now forced upwards (figure 3), so compressing the core 26, which in turn applies hydraulic pressure to the sleeve 28 so that the latter tends to expand radially outwards. the interface between the sleeve 28 and the workpiece 12 has a coefficient of friction the value of which is either unity (1) or very close to it, so that - given that the applied forces are such that the metal of the tube 12 is in an appropriately ductile state, and given also the effective absence of friction between the tube 12 and the mould the material of the tube tends to move with the sleeve 28, so that they expand radially, and also shorten axially, simultaneously with each other. when the radial expansion is complete, as shown in Figure 3, the ends of the tube 12 have therefore moved towards each other.

Actual radial expansion of the sleeve 28 and tube 12 does of course only occur locally, ie into the mould cavity 18. Suppose now that, in the relaxed condition of Figure 4, the sleeve section 29 lies opposite the cylindrical central zone 19 of the cavity 18 in Figure 1, so that this section is expanded radially towards the zone 19. As the rubber of the sleeve expands, the fibres of its reinforcement 32 become progressively reorientated, so that the -angle made with the plane 36 increases from its value a seen in Figure 4. Figure 5 shows the sleeve section 29 when it has expanded to a point where the new value p of this angle is 45 . Comparison between Figures 4 and 5 shows that the surface area of the section 29 has increased.Thus the reinforcement allows the rubber to expand, though it should be noted that Figure 5 represents the maximum possible expansion where the fibres of the reinforcement are inextensible. In practice the angle a is preferably so chosen that p does not reach 45 , thereby avoiding any risk of incomplete expansion of the tube 12 into the cavity 18.

It will be noted that the section 29 remains rectangular throughout its expansion, being axially shortened (parallel to the plane 36) in a relationship with the increase in its circumferential width which is predetermined by the constraints placed upon deformation of the rubber by the reinforcement 32. In this way the latter controls the deformation of the sleeve 28 towards the zone 19. The same principle of deformation control applies in those zones of the sleeve which are expanded out of their initial cylindrical shape, eg in the generally flared portions of the cavity 18 flanking its central zone 19, save that a section such as 29, if initially rectangular, will of course assume a different shape in these zones.

Immediately expansion has been completed, the ram is rapidly withdrawn, allowing the core 26 and sleeve 28 to relax so that the sleeve tends to become separated from the workpiece 12. The mould 16 is then opened and the workpiece removed. If the operation has been carried out hot, any post-heat treatment necessary to counteract the effects of annealing during the swaging process can then be carried out in a known manner.

It is found that when the process is performed under the above conditions, there is substantially no reduction in the wall thickness of the workpiece 12, even at bends of relatively sharp radius such as those seen in Figure 3.

The fibres of the orientated reinforcement in the sleeve 28 are themselves substantially incompressible and inextensible. while a woven or braided fabric 32 is also inextensible in the direction of both the warp and the weft, it can be made so that it can expand in at least the directions in which deformation is to be imposed. It will be realised that in order to preserve at least the sleeve 28 for further use, the operation, when performed hot, needs to be fast so that the sleeve suffers as little as possible from the effect of the heat. The process is found to lend itself to such rapid operation.

Hydraulic pressure may be supplied conventionally by a gas or hydraulic fluid. This is supplied, as shown in Figure 6, from a source 50, through a pump 52 and a pressure regulating valve 54, and via a suitable bottom plate 55 of the mould 16 into a chamber defined by the hollow interior of the sleeve 28. In this case of course, the top plate 20 and bottom plate 55 must be provided with suitable seals, such as spring loaded sealing plates 56, to prevent the fluid from penetrating between the sleeve 28 and the workpiece 12, or between the latter and the mould.

Claims

1. A process for radially expanding to a predetermined profile a portion of a tubular metallic workpiece, having a stress/strain characteristic in which it is plastic at tensile failure, the process including: - inserting a plunger comprising a cylindrical sleeve element into the workpiece, to extend through said workpiece portion with the sleeve element adjacent to the workpiece bore, the sleeve element being of rubbery material and including an orientated fibrous reinforcement such as to control the deformation of the sleeve element during subsequent radial expansion thereof, such that the latter is axially shortened in a predetermined relationship with increase in its diameter; - inserting the workpiece into a hollow mould having an internal profile corresponding to the said predetermined profile; and - applying hydraulic pressure to the workpiece bore through the plunger sleeve element, under conditions such that friction between the workpiece and the mould is substantially smaller than that between the workpiece and the sleeve element, thereby causing the sleeve element and workpiece together to expand radially and shorten axially until the profile of the workpiece conforms with that of the mould.

2. A process according to claim 1, including heating the workpiece, the hydraulic pressure being applied subsequently while the workpiece is hot.

3. A process according to claim 2 in which the workpiece is of aluminium or an aluminium alloy, wherein the workpiece is heated to about 3500C.

4. A process according to any one of claims 1 to 3, including lubricating the internal surface of the mould so as substantially to eliminate any friction between mould and workpiece.

5. Apparatus for radially expanding to a predetermined profile a potion of a tubular metallic workpiece, the apparatus comprising; - a hollow mould having an internal profile corresponding to the said predetermined profile; - a plunger comprising a cylindrical sleeve element of rubbery material, including an orientated fibrous reinforcement such as to control the deformation of the sleeve element during radial expansion thereof under internal hydraulic pressure such as to cause axial shortening of the sleeve element, by controlling the relationship between shortening and radial expansion of the sleeve element in a predetermined manner, the sleeve element being adapted for insertion in a said workpiece; and - a pressurising means for applying such internal hydraulic pressure to the sleeve element, whereby to deform such a workpiece in frictional contact with the sleeve element by simultaneous radial expansion and axial shortening of the said workpiece portion and sleeve element together.

6. Apparatus according to claim 5, wherein the rubbery material is a natural or synthetic rubber.

7. Apparatus according to claim 5 or claim 6, wherein the rubbery material is one having a coefficient of friction of substantially unity with the workpiece.

8. Apparatus according to any one of claims 5 to 7, wherein the reinforcement is made of substantially inextensible fibres.

9. Apparatus according to any one of claims 5 to 8, wherein the reinforcement is orientated in two directions.

10. Apparatus according to claim 9, wherein the two directions are respectively clockwise and anti-clockwise about the axis of the sleeve element.

11. Apparatus according to claim 9 or claim 10, wherein the reinforcement is of substantially inextensible construction, the sleeve element being a hollow cylindrical sleeve member.

12. Apparatus according to any one of claims 9 to 11, wherein the reinforcement is of woven or braided construction.

13. Apparatus according to any one of claims 9 to 11, wherein the reinforcement is of helically wound construction.

14. Apparatus according to any one of claims 9 to 13, wherein the reinforcement is of two-ply construction.

15. Apparatus according to any one of claims 5 to 14, wherein the plunger further comprises a core of rubbery material within the sleeve element, the pressurising means comprising the core and means for applying endwise pressure to the core to cause the latter to exert the said hydraulic pressure.

16. Apparatus according to claim 15 when dependent on claim 11, wherein the core is a separate member from the sleeve member and is of a different rubbery material.

17. Apparatus according to claim 15 or claim 16, including a rigid coaxial support bar extending through the core.

18. A process for radially expanding to a predetermined profile a portion of a tubular metallic workpiece, substantially as described herein with reference to the accompanying drawings.

19. Apparatus for radially expanding to a predetermined profile a portion of a tubular metallic workpiece, substantially as described herein with reference to Figures 1 to 5 of the accompanying drawings.

20. Apparatus according to claim 19, modified substantially as described with reference to Figure 6.